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Old 11th April 2009, 05:26 PM   #5201
Pano is offline Pano  United States
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Quote:
Originally posted by Teh
Pano...you're kidding right?
With all Earl's brought to these discussions you can say something like that?
I was kidding - a bit. It was meant as hyperbole. It missed the mark.

My apologies to the good doctor if it was harsh. But my reaction at reading it was certainly shock and disbelief.
Why? Because it so contradicts my 30 +years of experience with speaker cabinets. And the experiences of many others I know and respect.

So coming from someone as seemingly knowlegable as Dr. Geddes, well, I just don't know what to think.

So again, sorry if I stepped on toes. I apologize. But certainly my frame of reference has now changed - in a big way.
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Old 11th April 2009, 05:27 PM   #5202
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Rudolph - Never mind me; do you read ?
John records the impulse response and does a transform in order to chart the equivalent amplitude response variation with frequency.
So yes John.
I already appreciate how you get your results.

Maybe if you did a continuos recording in time over several sine cycles your trace would behave exactly as I claim ?
It is so long since I checked, but I believe Linkwitz observed amplitude responses in Time.

Cone amplitude linearity close to and where there is any system resonance is NOT the same for 360 to 540 degrees as it was for 0 to 180 degrees of a continuous waveform.
The same applies with music, especially when percussive.

What I have related also applies to U-frames when compared to a plain open baffle, and I do not regard 'time smear' as being a correct description because what is happening is a modification of the amplitude response in music time.

Cheers ........ Graham.
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Old 11th April 2009, 07:07 PM   #5203
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Default Not to step on toes, or threadjack since this is REALLY about Lynn's project right?

Dr Geddes built very elaborate cabinets from resin and carbon fiber for his highly acclaimed Summa loudspeakers and now builds everything out out of MDF because for him it is good enough.

Since he has in the past gone to the effort (and expense) to build fancy cabinets, I think he would say if it was necessary. There are a host of other things that are more important to him in the grand scheme of things if you read his posts.

As this is diy, build cabinets however you like with whatever you can afford. MDF, BB, Corian, phenolic laminate, aluminum, translam, aerolam, CLD....and brace and stuff it with as little or as much stuff as you want. No one can give you a definitive answer on boxes.
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Old 11th April 2009, 10:45 PM   #5204
gedlee is online now gedlee  United States
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Quote:
Originally posted by john k...


Yes, exaclty, this is what is meant by "transparent". That is, the effects of the box resoances alter the mechanical motion of the cone such that these resonances, or there effect on the cone, are then re-radiated from the front side. Perhaps it is the term transparent that is the source of confusion which is why I tried to explain (9define) it a while back.

The terms transparent and opaque are just lables being applied to indicate that either the cone reacts mechanically to the internal resonances, thus their effect impacts the radiated sound, or the cone does not.

John

Well once again it is the deffinition here that is where we differ, because to me the acoustic effect ON a cone is not the same thing as it being acoustically transparent. No sound need go through the cone for the box to have an effect on the cone motion, so to me THIS type of cone is NOT transparent. Given your deffinition (which I certainly do not agree with), cones are ALWAYS acoustically transparent, of course, but then there is no way to seperate the discussion between sound that transmits through the cone and sound effects that result from the cone acoustically coupling to the box. And they are quite different effects.

As to Wilson, I would say this. I believe that they use very dense box material because it is the the easiest way to get very high gloss FLAT surfaces. MDF just does not hold a flat enough surface when it is highly pollished (been there, done that). So I am suggesting that Wilson use Material-X because it takes a better finish. But of course there is no marketing slant to that, so they make one that everybody eats up. I've heard Wilsons often and I am not impressed, so would this make me believe that their uber rigid and dense boxes was a "major factor"?
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Old 11th April 2009, 11:14 PM   #5205
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I have some questions/observations about john k's graphs.

1) What are the dimensions of the open baffle?

2) The frequency markings of the first two graphs line up exactly, but the graph for the open baffle is stretched - why?
Is different smoothing used for the open baffle graph?

3) The chosen box volume is small for the 8" ScanSpeak 8554 - what is the Qtc of the test box?
Rough calculations put Qtc well over 1 - which will result (by design) in uneven bass response.

4) The chosen box is almost cubic (7" x 7" x 8") - why?
This is the worst possible shape for resonances. Would people really use a cubic sealed box for this driver? I don't think so.
I am also yet to see a U-frame open baffle design employing cubic dimensions.

5) Hands up all those who use boxes without any damping?

It appears to me that unrealistic, "worst case" design choices are being compared with open baffle.

Cheers,

Alex
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Old 11th April 2009, 11:55 PM   #5206
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Me think that the discussion on bafflesa and boxes is getting as complicated as a girls pigtails. Box panel resonance, how resonance get excited, cone transparency, significance of effects...
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Old 12th April 2009, 12:22 AM   #5207
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Quote:
Originally posted by Alex from Oz
I have some questions/observations about john k's graphs.

1) What are the dimensions of the open baffle?

2) The frequency markings of the first two graphs line up exactly, but the graph for the open baffle is stretched - why?
Is different smoothing used for the open baffle graph?

3) The chosen box volume is small for the 8" ScanSpeak 8554 - what is the Qtc of the test box?
Rough calculations put Qtc well over 1 - which will result (by design) in uneven bass response.

4) The chosen box is almost cubic (7" x 7" x 8") - why?
This is the worst possible shape for resonances. Would people really use a cubic sealed box for this driver? I don't think so.
I am also yet to see a U-frame open baffle design employing cubic dimensions.

5) Hands up all those who use boxes without any damping?

It appears to me that unrealistic, "worst case" design choices are being compared with open baffle.

Cheers,

Alex

1) These are all near field measurement baffle dimension makes no difference, but it was the same in all cases.

2) Accident. I reset the frequency limits to 10K for the OB and then when I re-opened the U and SB files for plotting I didn't notice the upper limit reset to 20K. I could show you the plot for the OB to 20k but it would look just like the others.

3) The intent was to show coupling of the box/U-frame resonances to the driver and how this affected radiated sound. I'm not trying to "build the correct box".

4) again the intent was to show the coupling. As for the U-frame, the resonances are basically all axial modes and are controlled by the length dimension. How long/short would you like it?

5) Again, no damping because the intent is to show the coupling. And don't be so impatient. I have a set of measurements where the U is damped different amounts to show what happens as damping is added.


The point here was to show what you have to deal with when building a U-frame or sealed box.

If Earl rather call this coupling that's fine with me. As far as I am concerned the labels don't mean much. Put a driver in an enclosure and the resonances of that enclosure may alter the driver's motion may color the sound. The degree to how much the could is colored will depend on how well the box resonances are damped. If you like, you can even consider the low frequency alignment of a woofer in a sealed box as a coloration since it alters the driver motion. However, I will stand by what I said some time ago and point out that these "coupling effect" which alter the cone motion generating uneven frequency response are minimum phase (with respect to the cone motion in the pistonic range) and can be corrected with MP eq. In fact, box damping is nothing more than acoustic eq.
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Old 12th April 2009, 02:26 AM   #5208
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This thread has to take the cake for windage...


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Old 12th April 2009, 04:06 AM   #5209
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Oh no, there are worse... :P
(but we are trying!)
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Old 12th April 2009, 08:27 PM   #5210
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Quote:
Originally posted by gedlee




As to Wilson, I would say this. I believe that they use very dense box material because it is the the easiest way to get very high gloss FLAT surfaces. MDF just does not hold a flat enough surface when it is highly pollished (been there, done that). So I am suggesting that Wilson use Material-X because it takes a better finish. But of course there is no marketing slant to that, so they make one that everybody eats up. I've heard Wilsons often and I am not impressed, so would this make me believe that their uber rigid and dense boxes was a "major factor"?

Do you have any proof that Wilson does this? A good friend of mine has worked for Wilson in the past as a consultant and he just laughed at the idea that Dave picks his material for finish rather that for good sound. There is an article available on the web that talks about how Wilson built two pairs of WATT’s, one pair out of his X material and one pair out of MDF. The writer said that there was a clear sound difference between the two pairs with the X material being far better. Also the two pairs did not have a finish on them. Another friend of mine a few miles away has a pair of Wilson’s largest speakers and they do not vibrate to the touch and have the best bass reproduction of any box speaker that I have heard.

Most of the readers of this thread are wanting to have the best sound possible and they are doing the work themselves to have better speakers and a lot of them are using the “no box” design. Other than having no box the only way to have less box acoustic output is to make it as dense and “dead” as possible.
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